Intellectually, a man knows that the size of his penis shouldn’t be specifically relevant in a relationship, to him or to a woman. His common sense tells him that it will certainly not be the major or controlling factor in a woman’s response to him. And yet … he can’t help believing that it is.

The most frequent question on all Internet Q-and-A sex sites continues to be, "Is size important?" A downloadable chart of four outline drawings (“low average” to “extraordinarily large”) can be found on the net, which a man can print out and use as a template against which to judge himself. Even the most balanced of men is capable of half-believing he is under-endowed. Many American men, according to the Kinsey Institute, believe the average erection is 10 inches—this despite (or because of) frequently accessing Internet pornography in which participants have shaved off their pubic hair to increase visibility and many have used a vascular device to pump up temporarily. But there should be solace for the average man in knowing that he is statistically within touching distance, as it were, of some 90 percent of all his fellows.

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When the Kinsey Institute reviewed its founder’s data 30 years after it was published, in the light of subsequent findings, it showed that 1 in 100 men reaches beyond the 5- to 7-inch erectile median to 8; that 7 in 1,000 men go beyond 8; and only 1 in 1,000 touches 9. But Durex and the Definitive Penis Internet surveys, while stressing that their core findings are consistent with Kinsey, have cautiously proposed that there are more very big penises—between 4 and 7 in every 100 men reaching 8 inches, between 30 and 40 in every 1,000 reaching 9, and between 10 and 30 in 1,000 reaching beyond. And where the institute’s data showed that erections above 9 inches are so rare (a word, incidentally, that Kinsey himself always used rather than “big”) as to be statistically immeasurable, both surveys have suggested that 1 in 100 men posts double figures. In the round, the institute found that 18 in 1,000 men have an erection over the median; Durex and the Definitive Penis propose this figure to be between four and eight times greater. Could Kinsey have been so wrong?

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Flaccid, the penis of the gorilla and the orangutan, both with much bigger bodies than man, is virtually invisible.

The problem for researchers has been that they have had to rely on participants providing their own measurements. The bulk of Kinsey’s data came from self-measurements (marked off on the edge of returned postcards); all the data in the Durex and the Definitive Penis survey undertakings were collected in this way—the DPS giving the average erection as 6.3 inches, with Durex giving it as 6.4. Are penises, then, like people, getting bigger? If men’s ears have pricked up at this point, the answer is no: The depersonalised and anonymous nature of the Internet almost certainly explains the apparent increase. Not that Durex and the DPS have not taken safeguards against humorists and delusionists. Durex eliminates extreme replies: lengths under 75 mm (3 inches), “the size of a large chilli”, and those over 250 mm (a touch under 10 inches), “the size of a large cucumber,” The Definitive Penis Survey has disregarded the blatantly fraudulent (“17-year-old lawyers and those claiming American Zulu warrior ancestry”) and eliminates the bottom 1 percent and the top 2 percent of replies; additionally the website has asked participants to provide an electronically transmitted photo which includes a tape measure.

Averaging the averages of Kinsey from over half a century ago, his institute’s from 25 years ago, and the Durex and Definitive Penis surveys from the last year of the millennium (only three-tenths of an inch apart, top to bottom, after all) we arrive at 6.25 inches, with a circumference of just under 5 inches being pretty consistent in all surveys; and that surely seemed as definitive as you can get, except that in 2001 Lifestyle Condoms (on the same mission as Durex) carried out the only large-scale study not to rely on self-measurements—and turned the penis issue on its head. After getting 300 volunteers to submit their aroused manhood to the attention of two tape-wielding nurses under the constant supervision of a doctor, Lifestyle reported the average erection to be 5.8 inches—about one-half an inch less than the above averaged averages. It’s worth noting that five years earlier two small-scale studies (one in Germany, one in Brazil) had pharmacologically induced erections in volunteers and both had averaged out at 5.7 inches. Even more startlingly, the same year the Journal of Urology had published the findings of a study in which 80 normal men of various ethnicities had also been pharmacologically aroused (the object in this case was ultimately to help in counselling others considering penile augmentation)—and arrived at an average of 5.08, almost three-quarters of an inch less than Lifestyle’s.

The medical profession continues to measure penises; between 2007 and 2010 at least 15 different studies were published, all of them hands-on. What now seems to be the focus of attention is the likelihood that men who know or think they are below average are unlikely to volunteer to be sized up, or allow themselves to be, meaning that averages could be lower than those recorded (allusion to which hypothesis might, in some circumstances, stand a small-penised man in good stead). What is incontrovertible is that where men and their penises are concerned there are lies, damned lies, and self-measurements.

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Objectively, even big human penises are small, other than in comparison with other human penises; but virtually all human penises are big in comparison with those of the other 192 primate species. Flaccid, the penis of the gorilla and the orangutan, both with much bigger bodies, is virtually invisible; erect, it reaches 1.5 inches or less; the chimpanzee, man’s closest relative (sharing 98 percent of his DNA) achieves an erection twice that of the other two apes but still only one-half the average human one. Why, comparatively, man’s penis is so disproportionately large is a question that engages a clutch of disciplines including archaeology, anthropology, and zoology, as well as the evolutionary, psychological, and sociological branches of biology. Collectively they remain at a loss to provide what is known as “ultimate causal explanation.”

The consensual “ological” view is that when man’s hominoid ancestors came down from the trees 4 million years ago, their penises were of a size with the apes—“vanishingly small,” according to archaeologist Timothy Taylor in The Prehistory of Sex. Then, however, when upright walking swivelled the sexual focus from rear to front of both sexes, a focus that was intensified by the loss of the majority of bodily hair other than in the genital area, the penis began the process of “runaway selection.”

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"In the round, the institute found that 18 in 1,000 men have an erection over the median" I'm not denying it's fun, but is it really worth the risk of having it get run over? More...

Feminists incline to the view that it happened because females wanted it that way; that when femina became erecta, the angle of the vagina swung forward and down, moving deeper into the body, obliging the penis, as Rosalind Miles put it in The Women’s History of the World, to follow the same principle as the giraffe’s neck: “it grew in order to get to something it could not otherwise reach.” On the other hand, the big penis may have evolved because that’s what possessors wanted—a greater attractant to potential mates and a more visible means of warning off rivals. A big penis also increased the male’s chance of inseminating a female who was having sex with other males, by getting closer to the cervix. There are objections to such theories—not least that other primate males have continued to propagate their species with considerably less at their disposal. As to the theory that the penis grew to assist humankind’s imaginative variety of sexual positions, orangutans and chimpanzees, particularly the pygmy chimpanzee or bonobo (a separate species, found in the Congo, which has a more upright gait and a more “human” skeleton), are equally imaginative in their coupling—and they can do it swinging from trees while man only talks about doing it swinging from chandeliers.

But if science cannot say definitively why man’s penis is so big, it does have an explanation as to why his testicles are the size that they are.

In the early 1980s the evolutionary psychologist David Buss caused widespread excitement among the “ologies” with the hypothesis (in The Evolution of Desire) that the more promiscuous a primate species, the larger the testicles of the males belonging to it—penis size, he surmised, was less relevant in achieving impregnation of a female having sex in rapid sequence with other males than being able to produce the most copious and frequent ejaculate. Subsequently, British scientists weighed the testes of 33 primate species, including man, to assess the testicle-promiscuity link. Interestingly, by this measure, the human male, the primate with the biggest penis, was not the king of the swingers: His testicles, together weighing 1.5 ounces, bore no comparison with those of the chimpanzee, which weighed an astounding 4 ounces, a three-times higher testes-to-body-weight ratio than humans. And the mighty gorilla, the primate with the smallest penis? Again he trailed the field, his testicles little more than one-half the weight of man’s. As Buss pointed out, the gorilla, with his monogamous harem of three to six females, faces no “sperm competition” from other males. On the other hand, the promiscuous common chimp has sex almost daily with different females and the even more promiscuous bonobo has sex several times a day.

Somewhere between gorilla and chimp comes man, neither entirely promiscuous nor entirely monogamous, his penis evolved far beyond those of his distant ancestors but his testicles or at least their firepower probably reduced— his sperm production per gram of tissue is considerably less than either chimps or gorillas, leading to the “ological” view that, as expressed by Lynn Margulis and Dorion Sagan in Mystery Dance: On the Evolution of Human Sexuality, he once, when the business of insemination was a contest, had a bigger “testicular engine.”